The present invention relates in general to guarded capacitance probe structures that are particularly useful for non-invasively measuring particle concentration and/or particle flow.
Capacitance probes have been used in the past to measure particle concentration or flows non-invasively. These probes are based on the principle that the capacitance between two conductive surfaces (e.g. electrodes) held at different voltages can be varied by changing the nature of a dielectric substance placed within the electric field generated between them. Since the dielectric constant of a particle containing liquid or gas varies depending upon the particle concentration, a capacitance probe can be employed to measure particle concentration non-invasively by positioning it on a vessel or pipe containing the liquid or gas to be analyzed, and causing the electric field generated between the probe's electrodes to pass through a portion of the liquid or gas near the vessel or pipe wall. As the particle concentration in the liquid or gas varies, so does it dielectric constant, and thus, the capacitance of the probe also varies. If two of the capacitance probes are placed a predetermined distance from one another on the vessel or pipe, their outputs can also be employed to determine the flow rate of the liquid or gas through the vessel by cross correlating the signals generated by each probe to determine the difference in time when portions of the flow stream having like dielectric constant pass each of the probes, and dividing the distance between the probes by this time difference.
Traditional capacitance probes have suffered from the possible interference of other charged surfaces in their vicinity and from considerable reductions in the signal-to-noise ratio. associated with the capacitance of a cable connecting the output of the probe to a necessary signal amplifier. In order to solve these problems, numerous probes have been devised that include a guard electrode in addition to the sensor and ground electrodes found in traditional probes. One such prior art probe is illustrated in FIG. 1. In particular, a probe 10 is shown that is positioned contiguous to the outside surface of a pipe 12 containing a fluid suspension 14 to be monitored. The probe 10 includes a centrally disposed ground electrode 16, a coaxial sensor electrode 18 and a guard electrode 20. The sensor electrode 18 is connected to a source of voltage (not shown) by means of a coaxial conductor 22 and generates a plurality of curved electric field lines 24 which pass through a portion of the fluid suspension 14 near the wall of the pipe 12 and terminate on the ground electrode 16.
As is typical, the guard electrode 20 surrounds the sensor electrode 18 everywhere except where the measurement is to be made. The guard electrode 20 is driven by a separate circuit, but its voltage is precisely matched to that of the sensor electrode 18. Consequently, no charge can accumulate in the cable between the guard and the sensor so that cable capacitance is effectively cancelled. The guard electrode 20 also shields the sensor electrode 18 from any charged surface in the vicinity of the probe 10 so that stray capacitances are also virtually eliminated.
Unfortunately, prior art guarded capacitance probes suffer from one significant drawback. In particular, a large portion of the vessel or pipe wall must also be held at the guard voltage in order for this geometry to work in a non-invasive wall probe. This constraint constitutes a significant impediment to industrial measurements.